The tri-block copolymer having poly-monovinyl aromatic monomer (typically styrene) block, poly-conjugated diene(typically butadiene) block and poly-monovinyl aromatic monomer block (abbreviated as A-B-A) is a kind of thermoplastic elastomer and has been widely used. The known processes for the preparation thereof include: two-step process which comprises first charging the initiator and styrene, and then charging styrene and butadiene after completion of the first polymerization step; coupling process which comprises first charging the initiator and styrene, charging butadiene after completion of the first polymerization step, and then charging a coupling agent after completion of the second polymerization step; or three-step process which comprises first charging the initiator and styrene, charging butadiene after completion of the first polymerization step, and then charging styrene after completion of the second polymerization step.
The results of application show the A-B-A copolymers still have some drawbacks in their properties, and therefore many attempts have been made to improve their properties by regulating their structures. For example, U.S. Pat. No. 4,104,330 disclosed a tetra-block copolymer having monovinyl aromatic monomer homopolymer block, conjugated diene homopolymer block, tapered monovinyl aromatic monomer/conjugated diene copolymer block and monovinyl aromatic monomer homopolymer block; U.S. Pat. No. 4,600,749 disclosed a multi-block copolymer having at least four blocks, that is tapered monovinyl aromatic monomer/conjugated diene copolymer block, monovinyl aromatic monomer homopolymer block, conjugated diene homopolymer block and monovinyl aromatic monomer homopolymer block. These copolymers are characterized in that in their molecular chains, there is further incorporated tapered monovinyl aromatic monomer/conjugated diene copolymer block in addition to monovinyl aromatic monomer homopolymer block, conjugated diene homopolymer block and monovinyl aromatic monomer homopolymer block, thereby forming multi-block copolymers having more than four blocks. As thermoplastic elastomer, many properties of these tetra-block or multi-block copolymers have been improved, but some properties are still not sufficiently desirable, more particularly, when the content of monovinyl aromatic monomer is higher, the elongation is lower, the hardness tends to be larger or the melt index and the strength are inappropriate, and therefore fail to meet the requirements of some application fields. Moreover, the procedures for preparing these block copolymers are complicated, and the polymerization lasts too long, which leads to a low production efficiency.